The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine with a common rail system, in which, during normal operation, the rail pressure is controlled by closed-loop control, and, when a load reduction is detected, a change is made from closed-loop control to open-loop control, wherein, during the open-loop control operation, the PWM signal is temporarily set to a PWM value that is higher than in normal operation in order to load the controlled system.
In a common rail system, a high-pressure pump delivers the fuel from a fuel tank to a rail. The admission cross section to the high-pressure pump is determined by a variable suction throttle. Injectors are connected to the rail. They inject the fuel into the combustion chambers of the internal combustion engine. Since the quality of the combustion is decisively determined by the pressure level in the rail, this pressure is automatically controlled. The closed-loop high-pressure control system comprises a pressure controller, the suction throttle with the high-pressure pump, the rail as the controlled system, and a filter in the feedback path. In this closed-loop high-pressure control system, the controlled variable is the pressure level in the rail. The measured pressure values in the rail are converted by the filter to an actual rail pressure and compared with a set rail pressure. The control deviation obtained by this comparison is then converted to a control signal for the suction throttle by the pressure controller. The control signal corresponds, e.g., to a volume flow in the unit of liters/minute. The control signal is electrically generated as a PWM signal of constant frequency, for example, 50 Hz. The closed-loop high-pressure control system described above is disclosed by DE 103 30 466 B3.
Due to the high dynamic response, a load reduction is an event that is difficult to control from the standpoint of automatic control engineering, since after a load reduction, the rail pressure can rise with a pressure gradient of up to 4000 bars/second. A passive pressure control valve that opens at a rail pressure of 1950 bars protects the common rail system from an impermissibly high rail pressure. If, for example, an internal combustion engine is being operated in a steady state at a constant rail pressure of 1800 bars, and a complete load rejection occurs, the time until the pressure control valve responds is 37.5 ms.
To improve the reliability of the closed-loop pressure control, DE 10 2005 029 138 B3 proposes that after a load reduction has been detected, the control operation be changed from closed-loop control to open-loop control. In the open-loop control operation, the PWM signal for activating the suction throttle is temporarily set to an increased PWM value by a step function, which accelerates the closing process of the suction throttle, and less fuel is delivered to the rail. After expiration of the timed step function, the operation reverts to closed-loop control. A load reduction is detected by virtue of the fact that the actual rail pressure exceeds a fixed limit. The method just described has proven effective for a complete load rejection, i.e., a reduction of the generator load from 100% to 0%.
In practice, however, it was found that the method is still not optimal in the case of a partial load reduction. A partial load reduction occurs when only some individual electrical consumers are deactivated. Under unfavorable conditions, pressure oscillations in the rail can arise, which are caused by several successive changes from closed-loop control to open-loop control with temporary PWM assignment.